Percentage of patients without an increase in HbA1c (≤ 0.3%) and with no Hypoglycaemic Events (HEs) [ Time Frame: visit 3 (anytime from week -4 to day -1 before start of Ramadan) to visit 4 (within 4 weeks post-Ramadan fasting period) (minimum 4.5 weeks to maximum 12 weeks) for HbA1c; and during 1 month (Ramadan) for HEs ] [ Designated as safety issue: No ]

Proportion of patients without an increase in HbA1c (≤ 0.3%) and with no Hypoglycaemic Events (HEs) [ Time Frame: visit 3 (anytime from week -4 to day -1 before start of Ramadan) to visit 4 (within 4 weeks post-Ramadan fasting period) (minimum 4.5 weeks to maximum 12 weeks) for HbA1c; and during 1 month (Ramadan) for HEs ] [ Designated as safety issue: No ]

Patients will be instructed to take Vildagliptin tablets at a fixed dose of 50 mg twice daily (double blind therapy)

Other Name: Galvus

Drug: Gliclazide

Patients will be instructed to take Gliclazide capsules at an equivalent dose to previous sulfonylurea in multiples of 80mg only (double blind therapy)

Drug: Metformin

Patients in both arms will take Metformin at a dose between 1500mg-2500mg per day, in an open label fashion

Drug: Placebo to Gliclazide

Patients will be instructed to take the Gliclazide matching placebo capsules at an equivalent dose to previous sulfonylurea in multiples of 80mg only (double blind therapy)

Drug: Placebo to Vildagliptin

Patients will be instructed to take Vildagliptin matching placebo tablets at a fixed dose of 50 mg twice daily (double blind therapy).

Study Arm (s)

Experimental: Vildagliptin + placebo to Gliclazide

Vildagliptin tablets will be given at 50mg twice daily (bid). Placebo to Gliclazide capsules will be given at an equivalent dose to previous sulfonylurea in multiples of 80mg only (80-320 mg/day). Patients will continue their open-label metformin therapy at dosage between 1500-2500 mg daily.

Interventions:

Drug: Vildagliptin

Drug: Metformin

Drug: Placebo to Gliclazide

Active Comparator: Gliclazide + placebo to Vildagliptin

Gliclazide capsules will be given in multiples of 80 mg (80-320 mg/day) at a dose equivalent to previous sulfonylurea dose, unless at the investigator's discretion it could be up-titrated to the next available dose (if HbA1c is higher than 7.5%). Placebo to Vildagliptin tablets will be given at 50mg twice daily (bid). Patients will continue their open-label metformin therapy at dosage between 1500-2500 mg daily.